PY106 Class17+18
Magnetic Flux
Magnetic flux is a measure of the number of magnetic field lines passing
through an area, A. The equation for magnetic flux, , is:
 = BAcos
where B is the magnetic field and  is the angle between the B vector
and the area vector, A.
Magnetic Flux and Faraday’s Law
The SI unit for magnetic
flux is the weber (Wb).
1 Wb = 1 Tm2
A
60o
A
90o
A
 = 0o
Maximum 
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Effect of Changing Magnetic Flux
 = 60o
 = 90o
Zero 
Figure. Illustration of how the magnetic flux through
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a coil varies with the angle .
Faraday’s Law
Faraday's Law says that the emf induced in a coil of N turns
is given by the rate of change of the magnetic flux in the coil:
Changing magnetic flux can induce an emf in a circuit, or
if the circuit is complete, it can induce an electric current
as illustrated below.
Faraday's Law:
  N

t
Since  = BAcos, the magnetic flux changes if any of B, A or
 changes. Faraday’s law has important applications in the
generation and distribution of electricity.
We call the voltage induced by a changing magnetic flux an
induced emf. In other words, changing magnetic flux acts like
a battery in a coil or loop, and a current flows when there's a
complete circuit.
The directions of current flow seen in (b) and (c) can be
deduced by Lenz’s law, which we will discuss shortly.
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Faraday’s Law (Cont’d)
As pointed out above, the magnetic flux changes or an emf is
induced if any of B, A or  changes. Below are some examples:
B changes:
Example where ____
A changes:
Example where ____

Example where ____
changes:
 = 0o
 = 60o
 = 90o
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1